1. Field of the Invention
This invention relates to lasers, and particularly to single frequency lasers such as those used for telecommunication purposes.
2. Description of Related Art
A single frequency, tunable laser with a narrow linewidth is useful for many applications. For example advanced sensors for defense applications require stable, highly single frequency lasers with as narrow a linewidth as possible. On the commercial front, optical networks can benefit from the added functionality that a tunable source can provide, and therefore the laser technologies required to support those networks continue to be a major area of focus of developers.
In order to provide single frequency operation, a variety of techniques have been used. One technique is to insert a Fabry-Perot etalon (FPE) into a laser cavity that is thin enough to restrict operation to a single mode within the gain-bandwidth of the laser material. However, this technique cannot be used effectively with broadband gain media due to the thinness that would be required to limit operation to a single frequency. Furthermore, the longitudinal modes (wavelengths) allowed by the etalon are set by its geometry, and therefore a laser with a conventional etalon is not tunable in any significant way.
It has been suggested to use birefringent materials in an intracavity filter configuration in order to reduce the number of longitudinal modes and to provide single frequency operation. In such conventional birefringent filters, a birefringent crystal is arranged within the laser cavity at Brewster""s angle, oriented such that the polarization is at 45xc2x0 between two differing dielectric axes. Problems with such conventional birefringent filters include lack of discrimination between adjacent modes; in other words, the peaks are not sharp enough to provide single frequency operation. In an attempt to improve discrimination, multiple birefringent filters (e.g. 2, 3, or more) may be used together to obtain single frequency operation; unfortunately this approach increases cost and complexity, and reduces reliability.
It may be noted that these two types of filtersxe2x80x94the etalon and the birefringent filterxe2x80x94have different uses. The FPE is generally used with a narrowband gain medium in an attempt to restrict oscillation to a single (or at most a few) frequencies, whereas the birefringent filter is generally used with a broadband gain medium to restrict oscillation to a narrower portion of the gain spectrum. For conventional single frequency lasers, a FPE is not constructed of a birefringent material, as this would not result in single frequency operation except under unusual circumstances.
Although both an etalon and a birefringent filter may be used simultaneously in a laser cavity in an attempt to restrict the oscillation of a broadband gain medium to a single frequency, that approach is unlikely to be effective by itself. Particularly, such an arrangement is highly unlikely to operate effectively over a significant tuning range since it requires that, at some point within the gain bandwidth, both the FPE and the birefringent filter have some preferred frequency in common; i.e. there is a requirement of synchronism between the preferred frequency of the FPE and birefringent filter. The existence of this synchronism is a fortuitous occurrence, although it can be controlled to some extent by independent control of some of the filter parameters, such as the angle of incidence or temperature of the FPE. Unfortunately, this arrangement is highly sensitive to any disturbance or other variation in the local environment. Furthermore, such an arrangement is extremely difficult to tune over any significant range.
Although single frequency lasers can be useful in a wide variety of wavelengths and applications, when developing photonic systems for communications, it becomes advantageous to consider the 1.5 micron wavelength regime as the band of choice. Use of this band allows system designers to leverage developments in the optical communications arena, usually leading to wider availability and lower product costs. This allows the use of low-loss optical fiber, filters, optical amplifiers, and so forth; all of which have been developed for the commercial marketplace. However, despite explosive growth in the number of optical products, there remains a significant shortfall relative to laser transmitters that meet the technical requirements for advanced military and commercial applications.
Many high performance applications involving 1.5 micron laser sources call for narrow linewidth and single-frequency output. In this context, system designers wishing to use conventional technologies are often forced to utilize the available semiconductor-based sources at 1.5 microns. Unfortunately, performance limitations of such semiconductor-based sources often require substantial design-arounds to meet system design goals. Semiconductor laser manufacturers achieve single-frequency, narrow linewidth operation by incorporating either distributed feedback (DFB) or distributed Bragg reflector (DBR) configurations into their basic semiconductor laser design. In this way, the DFB and DBR sections of the device enforce single frequency behavior, also leading to line narrowing. Although there have been great strides in improving the performance of these structures, typical DFB linewidths are still large (e.g. in the 1-10 MHz regime), prohibiting their use in applications that require very narrow linewidth emission. Additionally, by their very nature, DFB and DBR lasers are fixed-wavelength devices, and, as a consequence, are unsuitable for applications requiring rapid tunability.
One method currently utilized for producing a tunable output in the 1.5 xcexcm regime involves the use of conventional semiconductor-based lasers that have been incorporated into an external grating configuration. In such lasers, the external grating enforces single-frequency, narrow-linewidth performance of an otherwise multi-mode, broad-spectrum semiconductor laser. Tuning is achieved by mechanically tilting the grating. Although this method has been successfully implemented in commercial devices, the tuning rate is slow (on the order of seconds); a limitation which prevents their use in applications that require rapid tuning.
Another method being investigated to produce a tunable output employs a semiconductor-based vertical cavity surface emitting laser (VCSEL) gain region that is integrated with a micro-electro-mechanical (generically referred to as a MEMS device) mirror to provide one of the mirrors in the resonator. By moving the MEMS mirror along the VCSEL axis, the wavelength of the output can be tuned. However, this process also has relatively slow tuning (tens of microseconds). Furthermore, such devices have not been successfully brought to market.
To improve upon existing DFB and DBR laser capabilities by incorporating tunability, it has been suggested to incorporate additional sections into the DFB and/or DBR structures, such as disclosed in B. Mason, et al., IEEE Phot. Tech. Left., Vol. 10, No. 9, Sept. 1998 and in P. -J. Rigole, et al., Electron. Left., Vol. 32, No. 25, 1996. These devices integrate multiple frequency-selective sections into a common semiconductor laser structure. By varying the injection current into each of the independent sections, their frequency selective properties are slightly modified so as to produce wavelength tuning. Because of the relatively low amount of injection current required to tune across the desired wavelength range (typically on the order of 10 mA), the tuning speed can, theoretically, approach the tens-of-nanoseconds regime. In every case, however, the linewidths are wide which is typical of semiconductor DFB and DBR sources (on the order of one megahertz) with relatively low ( less than 10 mW) output power. In summary, although the DFB and DBR technologies appear to be progressing toward the development of fast switching devices, no products have been developed that are fast and have a narrow linewidth sufficient for present day system requirements.
Lasers based on erbium-doped fibers that emit directly in the 1.5 xcexcm regime present another alternative to achieving tunable output. Typically, these fiber lasers are pumped by single-mode diodes emitting in the 980 nm regime. The pump radiation is coupled into the core of the erbium-doped fiber by way of conventional fiber-coupling techniques. End mirrors that form the fiber laser resonator can be formed by either dielectric coatings or, more efficiently, fiber Bragg gratings (FBG""s) that are tuned to the wavelength of choice. Tunability is achieved by modifying the spectral reflectance of the FBG output coupler. By changing the effective periodicity of the FBG (typically by thermal or mechanical means), the output can be tuned across a frequency range that is consistent with the degree of change in the periodicity of the FBG. Although lasers of this type, as disclosed in Product info, MPB Technologies, Model EFL R98-TS, have produced narrow linewidth (10 kHz typical) and moderate output power (20 mW), the tuning rate is extremely slow (e.g. several seconds or more).
A laser is described in which a gain material having a broad emission spectrum includes a filter situated within the laser cavity that forces the laser to oscillate at a single frequency (i.e. a single longitudinal mode). Single longitudinal mode operation is useful for a wide variety of applications such as fiber optic telecommunications (both analog and digital), fiber optic-based RF transmission, and spectroscopic applications.
In one embodiment, a single wavelength laser is described that comprises a laser cavity including a first end mirror and a second end mirror, a broadband gain medium situated within the cavity, and a pump source for pumping the gain medium. The single frequency filter comprises a polarizer situated within the laser cavity, the polarizer defining a direction of polarization, and a birefringent element situated within the cavity. The birefringent element has a configuration including opposing partially reflective surfaces such as in an etalon configuration, and comprises a birefringent material arranged with two of its differing dielectric axes offset about 45xc2x0 from the direction of polarization.
To provide tunability, a wavelength control system is coupled to the birefringent element that may comprise a temperature control system including a thermoelectric cooler thermally coupled to the birefringent element and/or an electro-optic driver electrically coupled to the birefringent element.
In some embodiments the polarizer comprises a Brewster plate or a polarization-selective material. The birefringent element has a finite reflectance so that it can act as an etalon, and may be substantially uncoated, or may comprise a reflective coating.
An embodiment is described in which the intracavity filter comprises two birefringent elements of unequal optical length along the optical axis (xe2x80x9ca dual-BR filterxe2x80x9d), which advantageously reduces the voltage required to tune the frequency, thereby providing a wide tuning range within practical constraints. The first and second birefringent elements are arranged proximate to each other. Each includes parallel, smooth opposing surfaces normal to the optical axis, and each comprises a birefringent material. The first birefringent element is arranged with two of its differing dielectric axes oppositely aligned with the dielectric axes of the second birefringent element. Typically, the first and second birefringent elements comprise substantially identical materials. To provide tunability, the first and second birefringent elements are coupled to a wavelength control system, such as an electro-optic system.
In some embodiments, the single frequency filter can be implemented into a DPSS laser that includes a solid state gain medium optically pumped by a laser diode, an architecture that has many benefits. Typical characteristics of DPSS lasers include high output power, a near-diffraction-limited output from a circular beam, small physical size, frequency stability and the use of conventional diode pump sources. In addition, the individual optical components that comprise the basic DPSS laser typically are fabricated from materials that lend themselves to mass production techniques, such as xe2x80x9cslice and dicexe2x80x9d of the bulk materials and the application of dielectric coatings; both of which benefit greatly from mass-production scalability.
In one embodiment that uses a dual-BR filter as described herein, a single DPSS laser source can be tuned rapidly across a spectral range in the 1.5 xcexcm regime, which is consistent with conventional fiber-optic components, while providing a narrow linewidth output. This range can cover the 40 nm band typically referred to in optical communications as the erbium C-band (1.53-1.57 xcexcm). Moreover, in some embodiments random wavelength addressability can be achieved across this entire band in the tens-of-nanoseconds time frame. This enabling technology promises a dramatic increase in laser system design flexibility for both military and commercial applications.
Embodiments are described in which the filter comprises an external cavity that is arranged to select a single wavelength from a multi-wavelength laser beam applied to it. In such embodiments, the tunable filter for selecting a wavelength from an optical beam comprises an optical cavity, a polarizer situated within the optical cavity, the polarizer defining a direction of polarization, and one or more birefringent elements situated within the optical cavity. In an embodiment that includes a single birefringent element, the birefringent element has a configuration including opposing partially reflective surfaces such as in an etalon configuration, and the birefringent element comprises a birefringent material arranged with its dielectric axes offset about 45xc2x0 from the direction of polarization. A wavelength control system is coupled to the birefringent element. In some embodiments, the tunable filter comprises two birefringent elements.